1. Field of the Invention
The present invention relates to a CDMA (Code Division Multiple Access) Communication System, and more particularly to a frequency control method for controlling the frequency of channels established between the base station and a mobile station.
2. Description of the Related Art
In recent years, the CDMA communication method, which is effective in controlling interference and disturbance, is receiving increasing attention as a communication method used in mobile communication systems. In such a CDMA communication method, a user signal that is to be transmitted is spread by a spreading code on the transmission side to be transmitted, and the original user signal is obtained on the receiving side by using a spreading code identical to the spreading code for despreading.
In the CDMA communication system, a plurality of transmission sides effect spreading by using different spreading codes each having orthogonality, and at the receiving sides, each communication can be identified by selecting the spreading code which is used at despreading, thereby enabling a plurality of communications sharing the same frequency band.
However, since it is difficult to maintain absolute orthogonality between all of the spreading codes that are used, each of the spreading codes are actually not perfectly orthogonal and have correlation components with other codes. These correlation components become interference components in that communication and bring about deterioration in communication quality. This produces an interference component which increases with an increase in the number of communications.
In general, a certain signal-to-noise ratio is needed in a radio communication system to secure communication quality. For example, in a spread spectrum communication system, a certain signal-to-(noise+interference) ratio is needed to secure communication quality. This value is usually referred to as Eb/I0, where Eb is a desired reception wave power and I0 is an interference wave power. The Eb/I0 needed to secure the certain communication quality is also referred to as a required Eb/I0.
As described hereinabove, however, for one particular receiver, transmission signals sent to another receiver are noise components and generate interference to the desired signals transmitted to the receiver. As a result, Eb/I0 can be secured most efficiently when the powers of a plurality of transmission signals received at a receiver are all the same. For this reason, in a CDMA communication system, the base station controls the transmission power of each mobile station such that the Eb/I0 obtained from the communications of each of the mobile stations reaches a reference Eb/I0. The reference Eb/I0 is a value that is set based on a required Eb/I0, and is typically substantially equal to the reference Eb/I0.
Specifically, the base station instructs each mobile station to increase or decrease the present transmission power based on transmission power control signals for instructing the increase or decrease of transmission power to mobile stations contained in the downward channel transmitted from the base station to mobile stations.
However, when a failure occurs at a mobile station, the control of transmission power may be impossible, and in some cases transmission power continues to rise regardless of the transmission power control by the base station.
When this happens, the reception Eb/I0 of mobile stations other than the mobile station in which the failure occurred is degraded by the influence of the communication of the mobile station in which failure occurred, so that the reference Eb/I0 can no longer be secured. Therefore, the base station instructs each mobile station in which failure has not occurred to increase transmission power of the mobile station, and each mobile station performs communications at a transmission power greater than needed at each mobile station, thus shortening the battery life. In a worst case, all mobile stations within a cell communicate at the greatest transmission power that can be transmitted. As a result, the required Eb/I0 cannot be obtained at all mobile stations within the cell, making all mobile stations within the cell lose the ability to communicate.
In other words, the CDMA communication system of the prior art has suffered from problem that, should control of transmission power be lost at any particular mobile station and transmission power rise to an unnecessarily high level, transmission quality at other mobile stations within the cell can no longer be maintained.
It is an object of the present invention to provide a frequency control method in a CDMA communication system in which, even when a particular mobile station loses control of transmission power and hence an undesired interference wave is produced within the cell, this interference wave does not cause deterioration in the communication quality of other mobile stations in which failure has not occurred.
To achieve the aforementioned object, in the frequency control method in a CDMA system according to the invention, a CDMA base station determines that a particular mobile station has failed to control of transmission power if the difference between a reference Eb/I0 and the reception Eb/I0 of the communication channel that is established between the base station and the mobile stations has exceeded a predetermined value, and instructs the mobile station to turn off its power supply.
In the present invention, a CDMA base station instructs a particular mobile station to turn off its power supply in the event of a failure at that mobile station that loses control of transmission power, causing the transmission power of that mobile station to rise above the necessary level. The invention thus prevents deterioration in the communication quality of other normal mobile stations in which failures have not occurred due to the influence of transmission signals from the problem mobile station. In addition, the invention provides a solution to the problem of shortened battery life when a particular mobile station loses control of transmission power and other normal mobile stations communicate at transmission power greater than the necessary transmission power.
In another frequency control method in a CDMA system of the invention, a CDMA base station determines that a particular mobile station has failed to control of transmission power if the difference between a reference Eb/I0 and the reception Eb/I0 of the communication channel established between the base station and the mobile stations has exceeded a predetermined value, and instructs that particular mobile station to change the frequency used as the communication channel to another frequency.
In this invention, a CDMA base station instructs a particular mobile station to change the frequency used as the communication channel to another frequency when a failure has occurred at that particular mobile station, so that mobile station loses control of transmission power, and the transmission power of that mobile station rises above the necessary transmission power. The invention thus prevents deterioration in the communication quality of other normal mobile stations in which failures have not occurred caused by the influence of transmission signals from that mobile station.
In addition, according to another frequency control method in a CDMA system of the present invention, a CDMA base station determines that a particular mobile station has failed to control of transmission power if the difference between a reference Eb/I0 and the reception Eb/I0 of the communication channel established between the base station and the mobile station has exceeded a predetermined value, and instructs normal mobile stations other than that particular mobile station to change the frequency used as the communication channel to another frequency and also change the frequency used as the communication channel to the another frequency.
In this invention, when a failure occurs at a particular mobile station, so that the particular mobile station loses control of transmission power, and the transmission power of that mobile station rises above the necessary transmission power, a CDMA base station instructs normal mobile stations within the cell other than that mobile station to change the frequency used as the communication channel to another frequency and also change the frequency used as the communication channel to the another frequency. Accordingly, even when instructions from the base station can not be received by the mobile station that has failed to control of transmission power, deterioration in communication quality due to the influence of transmission signals from that mobile station can be prevented for other normal mobile stations in which no problems are occurred.
According to another frequency control method in a CDMA system of the present invention, a combination of any two or all three of the foregoing frequency control methods may be used.
According to the present invention, if the power supply of a mobile station in which a failure has occurred does not turn off despite instructions from the base station, the frequency used as the communication channel by that mobile station is changed; and if the frequency change is impossible, the frequency employed as the communication channel by other normal mobile stations and the CDMA base station is changed to another frequency. Thus, by using a method depending on the severity of the problem that has occurred in the mobile station that has failed to control of transmission power, the invention can prevent deterioration in communication quality in other normal mobile stations in which problems have not occurred caused by the influence of transmission signals from that mobile station.
The above and other objects, features, and advantages of the present invention will become apparent from the following description with reference to the accompanying drawings which illustrate examples of the present invention.